measurement immediately after.
We did not measure directly the beam diameter, but
a reliable estimate can be made on the following
grounds. According to , the standard diameter
of the beam in a JEM-100-CX-TEMSCAN microscope
operated in our microanalysis conditions is
about 6 nm. That was matching with another estimate
(5±10 nm) made for the beam diameter in our working
conditions . On the other hand, an approximate
measurement of the specimen drift made for each of
the analysed points showed that 70% of the drift
lengths ranged from approximately 0 to less than
100 nm. Therefore, by taking into account the upper
limit of 10 nm as the beam diameter for all our
measurements and the value of 100 nm as the drift
of all measured ``points'' (which is evidently an overestimation),
we can reliably estimate that 70% of our
measured Si/Al values correspond to a specimen surface
area of less than 103 nm2. This estimation can be
considered as including with a good approximation
also the increase of the analysed volume due to the
beam broadening effect in the specimen, which is
another unavoidable effect occurring in microanalysis
(see Fig. 1).
For the accurate determination of the Si/Al
ratio according to Eq. (1) it is necessary both to
measure as accurately as possible the X-ray peak
intensities and to know the actual value of the KAlSi
parameter of the spectrometer. The spectrometer
resolution power did not permit the complete separation
of the K
peaks corresponding to Al and Si, these
being always partially superposed. We had therefore
to use a carefully designed method for the peaks
integral evaluation, based on using only the Ærst
half-peak for Al and the second half-peak for Si.
Details of the procedure, which is not original, are
given elsewhere .
The accurate measurement of the KAlSi parameter
was performed by using as standards a series of
minerals of well-known Si/Al compositions. The
homogeneity of the Si and Al distribution at a micrometer-
scale was tested by several statistically consistent
measurements performed with a scanning beam
WDS X-ray spectrometer with an error less than 1%.
The calibration curve of our spectrometer is shown in
Fig. 2. The best linear Ætting curve corresponding to
these according to the least squares method was
calculated. In this way we determined for KAlSi the
value of 1.390.07. This is taken for calculating the
Si/Al weight concentration ratio according to Eq. (1).
The Si/Al weight concentration ratio was measured
on at least 50 randomly selected points, on randomly
selected particles, for each of the specimens listed in
Table 1. The results are shown graphically in Figs. 3±
7. The Ægures are grouped in a way similar to that used
for the table, so that a comparison between laboratory
prepared and industrial samples of similar composition
is easy. An exception is Fig. 5 corresponding to
laboratory samples for which no ``matching'' industrial
sample was available and where, consequently,
Fig. 2. Si/Al calibration curve of the EDS X-ray spectrometer.
C. SaÃrbu, B. Delmon / Applied Catalysis A: General 185 (1999) 85±97 89
Fig. 3. Nanoscale fluctuations of the Si/Al weight concentration ratio measured in specimens with a very low content of alumina (about 10%
alumina at the most). A is the value of the Si/Al ratio calculated on the basis of the global silica/alumina content. B is the average value
calculated on the basis of the Si/Al ratio values experimentally measured on all the analysed points (except for the extreme values that could
not be taken into account for calculation). See text for the physical meaning of the A value. Comparable specimens are shown on the same row
in all figures.
90 C. SaÃrbu, B. Delmon / Applied Catalysis A: General 185 (1999) 85±97
Fig. 4. Nanoscale fluctuations of the Si/Al weight concentration ratio measured in specimens with a low content of alumina (from 10% to
25% at the most). For the meaning of A and B see Fig. 1 and text.
C. SaÃrbu, B. Delmon / Applied Catalysis A: General 185 (1999) 85±97 91
Fig. 5. Nanoscale fluctuations of the Si/Al weight concentration ratio measured in specimens with a balanced content of alumina. Three
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